The present invention relates to radio communications technology and more particularly to direct conversion receivers and the architectures of such receivers.
At the present time the vast majority of radio receivers are of the superheterodyne type employing one or more intermediate frequency stages which allow for filtering and amplification at fixed frequencies. Alternatives have always existed to the superheterodyne architecture such as superregenerative and direct conversion designs. However, these alternative designs has been subject to serious flaws which have relegated radio receivers of these types to specialty roles within the radio communications world.
Despite the widespread adoption of the superheterodyne design, it has been widely recognized that the direct conversion architecture holds great promise for superior performance. For example, direct conversion receivers are not subject to image rejection problems and are not affected by cross-over spurious responses which are so often the cause of interference and related design difficulties in superheterodyne receivers. Further, direct conversion receivers feature simpler low pass type filters operating at audio frequencies in contrast to the often bulky and expensive bandpass filters employed in superheterodyne receivers, require only a single injection signal at one frequency rather than multiple signals at different frequencies (multiple conversion sets), and provide a good potential for the VLSI implementations since a majority of the receiver components consist of active audio and digital circuitry.
Referring now to FIG. 1, a prior art direct conversion receiver 10 would typically include an RF amplifier 2 and a splitter 4 for dividing an incoming RF communications signal into a pair of equal and inphase components. These RF components are mixed at the mixers 6 and 8 with separate injection signals on frequency with the communications signal but separated by 90.degree. in phase. Inphase and quadrature baseband components are thereby formed which are independently filtered and amplified at audio frequencies on separate signal channels 3 and 5 by separate filters 5 and 7 and amplifiers 9 and 11. The inphase and quadrature (baseband) components formed as a result of the mixing process allow the signal to be conveniently and accurately demodulated upon being supplied to a suitable signal processing unit 12.
This architecture works well except that variations between the signal channels which commonly occur as a result of changes in temperature, frequency and other operational parameters result in gain and phase mismatches which produce distortion products in the output of the receiver. Gain mismatches of as little as 0.2 dB and phase mismatches of as little as 1.degree. can result in 40 dB distortion products. While gain mismatches between signal channels can be reasonably controlled by careful selection of components, phase mismatches are much more difficult to regulate and constitute the primary design problem inherent in this architecture. The resulting distortion products can not ordinarily be reduced to less than 30-40 dB in practice and correspond to discrete tones which greatly limit the performance of the receiver. Researchers investigating the design of direct conversion radio receivers have frequently commented on this limitation, but none have heretofore been able to propose a practical method of overcoming gain and or phase errors arising within the direct conversion architecture.
It is, therefore, an object of the present invention to provide a direct conversion receiver of a new design which is characterized by superior performance due to the absence of distortion products arising from gain or phase mismatch errors between the signal channels in the receiver.
It is another object of the present invention to provide an architecture for a direct conversion receiver in which phase or gain errors between the signal channels within the receiver may be automatically corrected pursuant to simple signal processing algorithms.
It is a further object of the present invention to provide an automatic gain control system for use in conjunction with direct conversion receivers including those featuring the automatic correction of gain or phase errors between signal channels.
It is yet another object of the present invention to provide a signal filtering or selectivity system for use in conjunction with direct conversion receivers including those featuring automatic correction of gain or phase errors between signal channels.
It is a yet further object of the present invention to provide a new architecture for a direct conversion radio receiver which is economic to construct, provides superior performance and may be substantially implemented in VLSI.